This self-portrait of NASA’s Mars rover Curiosity combines dozens of exposures taken by the rover’s Mars Hand Lens Imager on Feb. 3, 2013 plus three exposures taken May 10, 2013 to show two holes (in lower left quadrant) where Curiosity used its drill on the rock target “John Klein.”

This image was taken by Mastcam: Left onboard NASA’s Mars rover Curiosity on Sol 69, shows a trench made by the scoop on Curiosity at “Rocknest,” the spot in Gale Crater where the mission’s first scoop sampling took place.

LA CAÑADA FLINTRIDGE >> The Curiosity rover has detected biologically useful nitrogen on Mars for the first time, 38 years after NASA initially discovered the chemical there, the agency announced Tuesday.

A Curiosity instrument detected nitric oxide when samples of loose sand and drilled bedrock were heated, suggesting that nitrates exist on Mars. Nitrates, a class of nitrogen molecules, can be used by living organisms.

“It’s called ‘fixed’ nitrogen, which means it’s been put in a chemical form that can be used readily for life,” said Ashwin Vasavada, Mars Science Laboratory project scientist at NASA’s Jet Propulsion Laboratory. “Most of the nitrogen on Earth and Mars would be in the atmosphere in N2, so two nitrogen atoms bound together to each other. That bond is extremely strong. It’s a triple bond, and life can’t make easy use of that strongly bonded form of nitrogen.”

On Earth, nitrogen makes up 78 percent of the air by volume. Humans use oxygen even though they mostly breathe in nitrogen because N2 is an inaccessible form, Vasavada said.

The “fixed” form, however, is required for all known forms of life because it is an ingredient for larger molecules like DNA and RNA, the genetic instructions for life, and amino acids, the building blocks of proteins. Nitrates are used to create hair and nails and to catalyze or control chemical reactions.

A white paper about this discovery was published Monday in the Proceedings of the National Academy of Science. Lead author Jennifer Stern, of NASA’s Goddard Space Flight Center in Maryland, examined samples from three places on Mars: windblown sand and dust at Rocknest and drilled mudstone from John Klein and Cumberland.

“Finding a biochemically accessible form of nitrogen is more support for the ancient Martian environment at Gale Crater being habitable,” Stern said in a statement. “Scientists have long thought that nitrates would be produced on Mars from the energy released in meteorite impacts and the amounts we found agree well with estimates from this process.”

The recent discovery adds to a body of evidence that suggests life may have inhabited ancient Mars: features resembling dry riverbeds and discovery of minerals that form only in the presence of liquid water.

“Fixed (nitrogen) is essential to terrestrial life, and this requirement has driven the evolution of metabolism on Earth,” the published study says. “The presence of fixed (nitrogen) on Mars suggests that, at some point, the first half of a nitrogen cycle was established.”

Living organisms probably didn’t create the recently discovered nitrogen molecules because the surface of Mars is inhospitable for all known forms of life. Stein’s team believes the nitrates are ancient and probably came from non-biological processes such as meteorite impacts and lightning events billions of years ago.

At Mars’ Gale Crater, NASA scientists have already found five key ingredients for life to exist: sulfur, hydrogen, oxygen, phosphorus and carbon. Now the agency has found the final player for life to occur: fixed nitrogen.

Life could have existed in Gale Crater 3.5 billion years ago because it had a habitable environment, Vasavada said.

“We actually acquired the sample that this study is based on almost two years ago. It took that long to make sure that the nitrogen in that special form — the fixed nitrogen — is not part of the contamination: We didn’t bring it with us. In other words, it required some pretty detailed work for us to confidently say that the nitrogen is from Mars.”

Correction: An earlier version of this article incorrectly stated life likely existed on Mars 3.5 billion years ago. Ashwin Vasavada actually said Gale Crater had a habitable environment 3.5 billion years ago.